Landfills have been the traditional waste disposal route for waste materials, including household waste and industrial waste. In the course of time many of the waste materials are degraded. However, with the huge increase in disposable packaging and disposable products presently available, the volume of waste materials to be disposed of has increased rapidly, and landfills, particularly in heavily populated areas, are filling rapidly with the increase in demand and new landfills are not being developed at a rate in keeping with the increased capacity needed. Further, the degradation process produces many compounds, such as salts, and elements, including hazardous metals, that can contaminate ground water supplies underlying the landfills.
Thus, alternatives to landfills have been investigated for some years. Recycling materials such as plastics, paper, metal cans and glass is feasible, and is being widely implemented. This reduces somewhat the volume of materials to be placed in landfills. However, the economies of recycling are not always attractive, and many communities have not yet embraced recycling because it is more expensive than landfills. Further, since recycling depends upon the average citizen for implementation, there are problems in gaining widespread compliance.
Another method of waste disposal is incineration. Incineration can reduce solid waste 90% by volume and 75% in mass, which is very attractive. However, incineration requires high temperatures to completely break down organic materials, and the gases generated must be scrubbed carefully to remove particulates and incompletely combusted organic materials to prevent them from entering the atmosphere. Thus, modern incineration units contain two combustion zones, a primary zone where solid wastes are burnt, producing bottom residues, and a secondary zone to complete the combustion and scrubbing of gases, producing air pollution control residue. The resultant completely combusted and scrubbed gases are cooled in heat recovery units, such as boilers and economizers, to produce steam, which in turn can be sold or sent to a turbine to produce electricity. These incinerators are known as resource recovery systems because they convert waste materials into energy.
Although the basic concept of resource recovery is highly attractive, these resource recovery systems have met with mixed success to date. There is widespread opposition to them on the part of local citizens, due to fears, well founded or not, that hazardous materials will be vented into the atmosphere.
In order to promote wider acceptance of these systems, they have been modified to improve scrubbing of flue or vent gases and to improve removal of particulates. More modern recovery incinerators use lime scrubbers and use various pollution control devices such as electrostatic precipitators and baghouses to remove acid gases and particulates.
Both types of residue produced in these incinerators contain heavy metals and salts. Bottom residue contains the heavier residues retained on the primary combustion grates, which include uncombusted materials such as metal pipe fragments, glass fragments and the like, as well as fragments from incompletely combusted combustible materials. Air pollution control residue is lighter particulate matter recovered from secondary combustion grates, and flue gas scrubbers, and they contain most of the more volatile and hazardous metals such as chromium, cadmium and lead, as well as various salts. These residues still must be deposited in landfills, with its concomitant leaching problems.
Studies have shown that the amount of metals such as chromium, cadmium and lead present in air pollution control residue from resource recovery units can exceed the standards set by the Environmental Protection Agency for disposal of hazardous waste materials.
In addition, the residues contain high amounts of soluble salts such as chlorides and sulfates. These materials are of concern because they too will leach into ground water, producing water with high dissolved salt residues, further contaminating water supplies.
Thus, it is now desirable and may become necessary to remove hazardous metals and salts from air pollution control residues before exporting the residue to landfills. The resultant non-toxic residue may be able to be reused by mixing with other solids, such as roadbed materials, further lessening the burden on landfills.
A method then of removing metals such as chromium, cadmium and lead from air pollution control residue and bottom residue, and soluble salts, prior to landfill disposal, that is feasible and inexpensive, would be highly desirable. However, a major problem in treating incinerator residues is the wide variation in the nature of the metals present in a particular residue, as well as their concentration, and the relative amounts of soluble salts present, depending on the incinerator source. The residues also vary in composition and characteristics from one batch to another from the same incinerator due to the variations in waste feedstock materials. These variations make it difficult to treat incinerator residues obtained at various times and/or from various sources in an efficient and continuous manner.
Thus, a method of treating incinerator residues from various sources and containing variations in metal and salt concentrations in a continuous and economical manner to produce residues that are environmentally safe would be highly desirable.